Aim Allochronic speciation occurs when sympatric populations sharing similar feeding regimes diverge because they mate at different times. Such speciation mode is fascinating because it permits the study of subsequent adaptations and/or geographic range shifts undergone by the populations that face new ecological constraints. Moreover, exploring whether intraspecific differentiation is accompanied by niche divergence is crucial for planning efficient biodiversity management and invasive species control. Here, we address the topic of potential climatic niche divergence between two sympatric populations of the pine processionary moth Thaumetopoea pityocampa (PPM) that diverged under allochrony. The larval development of a PPM population recently discovered in coastal regions of Portugal, referred to as the summer population (SP), occurs in the spring-summer instead of the autumn-winter as for all other populations of this species. Location Portugal. Methods We carried out intensive field sampling in the area where both populations are sympatric and calibrated spread (MigClim approach) and species distribution models (ensemble forecasting approach) to depict the response of SP to environment and predict its potential range under current and future climatic conditions. Results Since its discovery in 1997, the distribution of SP has been expanding along the Western coasts of Portugal. Still, its establishment in inland regions failed, even though suitable hosts occur. Our models identify maximal temperatures as the main constraint explaining SP absence from inland regions. The coastal area where winter populations and SP co-occur displays unique climatic conditions (moderate maximal and minimal temperatures), which enable the coexistence of these populations with totally different phenologies. The models predict a future decrease of SP range due to climate warming. Main conclusions We show here that a climatic niche shift occurred following allochronic divergence. This study highlights how climate differentially affects allochronic populations and how considering intraspecific diversity is crucial when predicting species responses to climate change.